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A few months ago I replaced the bottom bracket on my 2002 Fuji Finest. After reassembly I noticed the chain "wobbles" slightly when pedaling. You can see in this video the chain drifts toward the outer edge of the derailleur during rotation.

I assumed the issue is caused by one of two things: 1) the BB; 2) the crank.

Here is what I've attempted so far:

  1. I had another, new BB available. Thought maybe Bracket A was wonky. Both are Shimano square taper brackets new out of the box. I replaced bracket A with bracket B, reattached the arm. Still wobbles.

  2. I had a spare, new crankset available out of the box. Thought maybe Crank A was bent. I tried Cranks A and B on both brackets A and B (so combinations of AA AB BB BA). Still wobbles.

  3. I reseated both cranks on the BBs. The bolts are on tight and the crank itself feels solid. Still wobbles.

At this point I'm a bit stymied. I accept that the bike is rather old, and the bike still functions just fine (I've put a few hundred miles on it since the BB replacement). My main concern is the wobble may get worse if not addressed now.

Any thoughts on this? I appreciate the insight of the more technically adept!

  • Did each crank have its own chainrings or did you use the same rings with the different cranks? In the second case the rings could be out of true. – Carel May 1 at 20:09
  • Good question. Completely different chainrings, brand new with new cranks. – Tristan Kloss May 1 at 20:21
  • The "wobble" is pretty small. Is it possible either the fixed or adjustable cup may have been cross threaded? You can check for this by slowly rotate the crank and feeling if bearing play is loose or tight. In one position the bearing may feel tight and in another loose. You can also disassemble the crank and inspect the BB threads for damage. – P. Barney May 1 at 22:01
  • FWIW, I have an internal gear hub and when I got the bike, the shop tried out several chainrings until they found one that wouldn't wobble too much (though that was orthogonal to your wobble: it was rotating out of center). They told me that since derailleur gears are predominant and they anyways have a tensioner, the chainrings are not manufactured with very high precision. – cbeleites unhappy with SX May 2 at 20:27
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The amount of wobble shown in the video is fairly typical, for better or for worse, which is not to say it's a good thing or that the FD adjustment can necessarily be free of rub in all combinations and under variable load with it. You don't have to worry about this getting worse if you leave it alone.

For what it's worth, you can multiply your "AA AB BB BA" set by four, because the rotational position that a crank is mounted on a spindle determines how bad the total wobble is, and making things truly as close to perfect as obtainable requires you to test all of them. I don't recommend actually doing this in most cases, but it is true that one position will be the best, possibly to an extent that matters.

The nature of runout in a square BB spindle is that the mounting orientation 180 degrees different will distort the chainring the same amount the other direction, meaning that one orientation's error might stack with any imprecision in the crank in a way that maximizes the effect, but mount it the opposite way and the two could cancel each other out and look great.

For cranks with rings mounted on different sets of bolt holes, i.e. most triples but not most doubles, you can observe whether all the rings are moving in unison to draw inferences about whether a given bit of distortion is coming from the spindle or the crank. And of course, even on a double, you can compare what's happening between the two rings to infer whether you're looking at error in the spider tabs or the rings themselves.

Spider tabs can also be trued. Before doing so it's important to do some kind of testing or measuring to make sure it's not the BB or chainrings that are causing your problems.

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  • Thank you. Funny you mention the distortion between rings. This crankset is old enough to be a triple, which I removed the smallest cog. Tonight I noticed that the two remaining chainrings were distorted at the bolt that passes through the crank itself. I relieved some of the pressure on that bolt and that improved the distortion. After reinstalling the BB I'm convinced it must be the way the crank is mounted on the BB or the alignment of the chainrings. I'm looking at getting a different double crank soon and will probably not worry about this in the meantime. – Tristan Kloss May 2 at 2:36

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